A system has been explored in which a number of lines of Vigna sinensis (the cowpea or blackeye pea) are immune to a virus although most lines are susceptible. Natural immunity is being studied in the hope that clues for the development of antiviral strategies will become apparent as mechanisms of immunity are uncovered. Immune plants have proved, in most instances, to be sources of fully susceptible protoplasts. Therefore, the immunity does not seem to reside in the cytoplasm but external to it. We search for agents in the intercellular spaces, or other regions external to the cell membrane, that can bind or inactivate virus and thereby prevent it from reaching the cytoplasm. On line of cowpea is the source of protoplasts that are highly resistant to the virus. Present evidence indicates that the resistance is not fully expressed in cells not exposed to virus. A low level of virus replication seems to induce a response by the cells that prevents or restricts further replication. We are analyzing cells exposed to virus in a search for macromolecules that may be involved in the postulated response.